DOC/CLN: Revise + clean up index.md and 3 how-tos

[NickleDave]

• Make minor revisions/typo fixes etc in index.md
• Revise, add headers in doc/howto/remove-silent-labels-textgrid.md
• Clean up doc/howto/convert-generic-seq.md
• Make fixes in doc/howto/remove-silent-labels-textgrid.md

[codecov-commenter]

Codecov Report

Base: 94.37% // Head: 94.37% // No change to project coverage :thumbsup:

Coverage data is based on head (7c22a01) compared to base (c2f657d). Patch has no changes to coverable lines.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #217 +/- ## ======================================= Coverage 94.37% 94.37% ======================================= Files 39 39 Lines 1459 1459 ======================================= Hits 1377 1377 Misses 82 82 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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