Documentation/filesystems: sharedsubtree: add section headings

Several of the sections are missing underlines. This makes the
generated contents have missing entries, so add the underlines.

Fixes: 16c01b20ae05 ("doc/filesystems: more mount cleanups")
Fixes: 9cfcceea8f7e ("[PATCH] Complete description of shared subtrees.")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20230508055938.6550-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: filesystems: convert sharedsubtree.txt to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add it to filesystems/index.rst

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/6692b8abc177130e9e53aace94117a2ad076cab5.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>