docs: clarify on_attach example, quick start

[alex-courtis]

There is a little confusion around how to change the default mappings (briefly glimpsed on matrix chat). Added more examples.

Clarified quick start.

Build a user readable, linked table for mappings as an alternative to the default on_attach. Did not include it as I couldn't find a sensible place. Thoughts @gegoune ?

`<C-]>`           CD                      |nvim-tree-api.tree.change_root_to_node()|
`<C-e>`           Open: In Place          |nvim-tree-api.node.open.replace_tree_buffer()|
`<C-k>`           Info                    |nvim-tree-api.node.show_info_popup()|
`<C-r>`           Rename: Omit Filename   |nvim-tree-api.fs.rename_sub()|
`<C-t>`           Open: New Tab           |nvim-tree-api.node.open.tab()|
`<C-v>`           Open: Vertical Split    |nvim-tree-api.node.open.vertical()|
`<C-x>`           Open: Horizontal Split  |nvim-tree-api.node.open.horizontal()|
`<BS>`            Close Directory         |nvim-tree-api.node.navigate.parent_close()|
`<CR>`            Open                    |nvim-tree-api.node.open.edit()|
`<Tab>`           Open Preview            |nvim-tree-api.node.open.preview()|
`>`               Next Sibling            |nvim-tree-api.node.navigate.sibling.next()|
`<`               Previous Sibling        |nvim-tree-api.node.navigate.sibling.prev()|
`.`               Run Command             |nvim-tree-api.node.run.cmd()|
`-`               Up                      |nvim-tree-api.tree.change_root_to_parent()|
`a`               Create                  |nvim-tree-api.fs.create()|
`bmv`             Move Bookmarked         |nvim-tree-api.marks.bulk.move()|
`B`               Toggle No Buffer        |nvim-tree-api.tree.toggle_no_buffer_filter()|
`c`               Copy                    |nvim-tree-api.fs.copy.node()|
`C`               Toggle Git Clean        |nvim-tree-api.tree.toggle_git_clean_filter()|
`[c`              Prev Git                |nvim-tree-api.node.navigate.git.prev()|
`]c`              Next Git                |nvim-tree-api.node.navigate.git.next()|
`d`               Delete                  |nvim-tree-api.fs.remove()|
`D`               Trash                   |nvim-tree-api.fs.trash()|
`E`               Expand All              |nvim-tree-api.tree.expand_all()|
`e`               Rename: Basename        |nvim-tree-api.fs.rename_basename()|
`]e`              Next Diagnostic         |nvim-tree-api.node.navigate.diagnostics.next()|
`[e`              Prev Diagnostic         |nvim-tree-api.node.navigate.diagnostics.prev()|
`F`               Clean Filter            |nvim-tree-api.live_filter.clear()|
`f`               Filter                  |nvim-tree-api.live_filter.start()|
`g?`              Help                    |nvim-tree-api.tree.toggle_help()|
`gy`              Copy Absolute Path      |nvim-tree-api.fs.copy.absolute_path()|
`H`               Toggle Dotfiles         |nvim-tree-api.tree.toggle_hidden_filter()|
`I`               Toggle Git Ignore       |nvim-tree-api.tree.toggle_gitignore_filter()|
`J`               Last Sibling            |nvim-tree-api.node.navigate.sibling.last()|
`K`               First Sibling           |nvim-tree-api.node.navigate.sibling.first()|
`m`               Toggle Bookmark         |nvim-tree-api.marks.toggle()|
`o`               Open                    |nvim-tree-api.node.open.edit()|
`O`               Open: No Window Picker  |nvim-tree-api.node.open.no_window_picker()|
`p`               Paste                   |nvim-tree-api.fs.paste()|
`P`               Parent Directory        |nvim-tree-api.node.navigate.parent()|
`q`               Close                   |nvim-tree-api.tree.close()|
`r`               Rename                  |nvim-tree-api.fs.rename()|
`R`               Refresh                 |nvim-tree-api.tree.reload()|
`s`               Run System              |nvim-tree-api.node.run.system()|
`S`               Search                  |nvim-tree-api.tree.search_node()|
`U`               Toggle Hidden           |nvim-tree-api.tree.toggle_custom_filter()|
`W`               Collapse                |nvim-tree-api.tree.collapse_all()|
`x`               Cut                     |nvim-tree-api.fs.cut()|
`y`               Copy Name               |nvim-tree-api.fs.copy.filename()|
`Y`               Copy Relative Path      |nvim-tree-api.fs.copy.relative_path()|
`<2-LeftMouse>`   Open                    |nvim-tree-api.node.open.edit()|
`<2-RightMouse>`  CD                      |nvim-tree-api.tree.change_root_to_node()|

[gegoune]

LGTM. I think we should provide user readable list of default mappings coming from default_on_attach() as some users might rely on it and it's good for discoverability.

@hinell if you have any mapping related suggestion this is a good place for it.

[hinell]

LGTM

[alex-courtis]

LGTM. I think we should provide user readable list of default mappings coming from default_on_attach() as some users might rely on it and it's good for discoverability.

Stuck them in quick start. The mappings sections were too big...